1. Field of the Invention
The present invention generally relates to medical methods and apparatus. More particularly, the present invention relates to methods and apparatus used to restrict flexion of a spinal segment. The methods and apparatus disclosed herein may be used alone or in combination with other orthopedic procedures such as those intended to treat patients with spinal disorders. Exemplary spinal disorders include but are not limited to back pain as well as joint problems, vertebral fractures, intervertebral disc, nucleus and annulus problems.
A host of spinal conditions exist which often result in mobility issues and/or back pain. A major source of chronic low back pain is discogenic pain, also known as internal disc disruption. Discogenic pain can be quite disabling, and for some patients, can dramatically affect their ability to work and otherwise enjoy their lives. Patients suffering from discogenic pain tend to be young, otherwise healthy individuals who present with pain localized to the back. Discogenic pain usually occurs at the discs located at the L4-L5 or L5-S1 junctions of the spine. Pain tends to be exacerbated when patients put their lumbar spines into flexion (i.e. by sitting or bending forward) and relieved when they put their lumbar spines into extension (i.e. by standing or arching backwards). Flexion and extension are known to change the mechanical loading pattern of a lumbar segment. When the segment is in extension, the axial loads borne by the segment are shared by the disc and facet joints (approximately 30% of the load is borne by the facet joints). In flexion, the segmental load is borne almost entirely by the disc. Furthermore, in flexion the nucleus shifts posteriorly, changing the loads on the posterior portion of the annulus (which is innervated), likely causing its fibers to be subject to tension and shear forces. Segmental flexion, then, increases both the loads borne by the disc and causes them to be borne in a more painful way. It would therefore be desirable to provide methods and apparatus that reduce or modulate loading on the disc and adjacent tissue.
A number of treatments exist for addressing back pain and spinal mobility. Some of these include the use of artificial discs, artificial nucleus replacement, annulus repair, kyphoplasty treatment of vertebral fractures, instrumentation of the spinal segment with or without concomitant fusion, and facet joint replacement. Many of these treatments are promising, yet in some cases they also have potential drawbacks. For example, when spinal fusion is performed, excessive motion or loading of spinal segments may result. Often, the excessive motion or loading and consequent effects occur at a level adjacent the fusion (referred to as adjacent segment degeneration or junctional syndrome). This can result in further degeneration of the motion segments. Therefore, it would be desirable if flexion at adjacent level(s) of the spinal segment were restricted, thereby reducing or eliminating the excessive motion and any further degeneration.
Other treatments such as disc replacement, nucleus replacement, annulus repair, facet joint repair, and vertebral fracture repair could also benefit from restricted flexion in the vicinity of treatment area. For example, excessive flexion may loosen the purchase of a prosthesis to the anatomical structures, such as by toggling the pedicle screws that anchor a prosthetic facet joint device. Restricting flexion in the vicinity of the treatment area modulates loads borne by these implants or by surrounding tissue thus reducing, eliminating, or mitigating iatrogenic damage to tissue as well as reducing loads borne by any prostheses and adjacent tissue and further providing additional flexion stability.
For the aforementioned reasons, it would therefore be advantageous to provide methods and apparatus that modulate loads borne by implants used in spinal surgery. It would also be desirable if such methods and apparatus would also modulate loads borne by tissue in the vicinity of the surgically treated region and also provide additional flexion stability. It would further be desirable if the apparatus and methods providing the flexion stability preserved the natural motion and physiology of the patient so as to allow the patient to maintain mobility and minimize or avoid detrimental clinical effects caused by forces resulting from non-physiological loading. It would be further desirable if such methods and apparatus were minimally invasive to the patient, cost effective, and easy to use. It would further be desirable if such methods and apparatus were resistant to damage or failure over repetitive loading conditions in the body.
2. Description of the Background Art
Patents and published applications of interest include: U.S. Pat. Nos. 3,648,691; 4,643,178; 4,743,260; 4,966,600; 5,011,494; 5,092,866; 5,116,340; 5,180,393; 5,282,863; 5,395,374; 5,415,658; 5,415,661; 5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,562,737; 5,609,634; 5,628,756; 5,645,599; 5,725,582; 5,902,305; Re. 36,221; U.S. Pat. Nos. 5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,248,106; 6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; 7,029,475; 7,163,558; Published U.S. Patent Application Nos. US 2002/0151978; US 2004/0024458; US 2004/0106995; US 2004/0116927; US 2004/0117017; US 2004/0127989; US 2004/0172132; US 2004/0243239; US 2005/0033435; US 2005/0049708; 2005/0192581; 2005/0216017; US 2006/0069447; US 2006/0136060; US 2006/0240533; US 2007/0213829; US 2007/0233096; 2008/0009866; 2008/0108993; Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO2004/052246 A1; WO 2004/073532 A1; WO2008/051806; WO2008/051423; WO2008/051801; WO2008/051802; and Published Foreign Application Nos. EP0322334 A1; and FR 2 681 525 A1. The mechanical properties of flexible constraints applied to spinal segments are described in Papp et al. (1997) Spine 22:151-155; Dickman et al. (1997) Spine 22:596-604; and Garner et al. (2002) Eur. Spine J. S186-S191; Al Baz et al. (1995) Spine 20, No. 11, 1241-1244; Heller, (1997) Arch. Orthopedic and Trauma Surgery, 117, No. 1-2:96-99; Leahy et al. (2000) Proc. Inst. Mech. Eng. Part H: J. Eng. Med. 214, No. 5: 489-495; Minns et al., (1997) Spine 22 No. 16:1819-1825; Miyasaka et al. (2000) Spine 25, No. 6: 732-737; Shepherd et al. (2000) Spine 25, No. 3: 319-323; Shepherd (2001) Medical Eng. Phys. 23, No. 2: 135-141; and Voydeville et al (1992) Orthop Traumatol 2:259-264.